1. Field of the Invention
The present invention concerns a process for the compensation of contrast inhomogeneities in magnetic resonance images caused by spatial distributions of the radio frequency field associated with the radio frequency pulses that are emitted in order to excite nuclear spins in the subject. The invention also concerns a magnetic resonance measurement system with a contrast homogenization feature in order to compensate for such contrast inhomogeneities in acquired magnetic resonance images.
2. Description of the Prior Art
Magnetic resonance imaging, also called magnetic resonance tomography, is a technique that is now widespread for acquiring images of the body interior of a living object to be examined. In order to acquire an image with this method, the body or the body part to be examined must first be exposed to a homogenous static basic magnetic field (usually characterized as a B0 field), which is generated by a basic field magnet of the magnetic resonance measuring instrument (scanner). During the data acquisition for the magnetic resonance images, rapidly switched gradient fields for local coding are superimposed on this basic magnetic field, these fields being generated by gradient coils. Moreover, with radio frequency antennae, radio frequency pulses of a defined field strength are irradiated in the object to be examined. The magnetic flux density of these radio frequency pulses is usually termed as B1. The pulse-shaped radio frequency field is therefore generally also called the B1 field for short. By means of these radio frequency pulses the nuclear spins of the atoms in the object to be examined are excited in such a way that they are deflected by a so-called “excitation flip angle” (in the following also referred to as “flip angle”) from their state of equilibrium parallel to the base magnetic field B0. The nuclear spins then precess in the direction of the basic magnetic field B0. The magnetic resonance signals generated as a result are picked up by radio frequency receiving antennae. The receiving antennae can either be the same antennae with which the radio frequency pulses are irradiated, or separate receiving antennae can be used. The magnetic resonance images of the object to be examined are finally created on the basis of the received magnetic resonance signals. Each pixel in the magnetic resonance image is assigned to a small body volume, a so-called “voxel”, and every brightness or intensity value of the pixels is linked with the signal amplitude of the magnetic resonance signal received from this voxel. The relationship between a resonant irradiated radio frequency pulse with the field strength B1 and the flip angle a achieved with it is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic relationship, which for most magnetic resonance examinations is a fixed matter constant, and τ is the exposure time of the radio frequency pulse.
The flip angle achieved by an extended radio frequency pulse, and thus the strength or intensity of the magnetic resonance signal to be measured, thus depends not only on the length of the pulse, but also on the strength of the irradiated B1 field. The contrast and brightness of a magnetic resonance image are likewise dependent on the flip angle of the irradiated radio frequency pulse. Insofar as a spatial distribution of the amplitude of the radio frequency field and thus the flip angle exists, the result is an undesired dependency of the image contrast on the spatial position, since the intensity distribution caused by the field distribution overlays the intensity distribution of the measured parameters (such as, for example, the tissue material in each position), which contains the image information. This sort of undesired amplitude distributions of the radio frequency field occur due to the penetration behavior of the radio frequency field in dielectric and conductive media, particularly in zones of high magnetic field strengths of over 3 Teslas, or due to the use of local transmission coils or transmission arrays.
A reduction in this effect could be achieved by taking steps to homogenize of the irradiated radio frequency field in the examination subject. Yang et al. in Proc. Intl. Suc. Mag. Reson. Med 9, 2001, page 1096, under the title “Manipulation of Signal Intensity Distribution with Dielectric Loading at 7.0 T” suggest using dielectric pillows to homogenize the radio frequency field in the body. However, until now there has been no universally functional configuration applicable for every body and every position. A practical implementation of this concept is thus not currently foreseeable.
Alternatively, R. Deichmann, C. D. Good, and R. Turner, in “RF Inhomogeneity Compensation in Structural Brain Imaging” in Magnetic Resonance in Medicine 47, pages 398-402, 2002, suggest a measurement process which with respect to contrast is less sensitive to variations in the HF amplitude. This process, however, has the disadvantage that the necessary radio frequency pulses turn out to be very long and radio frequency-intensive. This increases the SAR (Specific Absorption Ratio), that is more stress is placed on the patient. Moreover, specific assumptions must be made about the penetration behavior, such as for instance an isotropic dielectric focus in the head, although it cannot be presumed that these assumptions apply in this form in any concrete examination.